The production of vinyl-coated synthetic yarns where the vinyl coating or encapsulation contains an unactivated foaming agent is well known. In general, the synthetic yarns are continuous filament or multi-filament yarns ranging from about 150 to about 2,200 denier, the yarns themselves generally being of nylon, rayon, glass or polyester. The synthetic core yarn is completely encapsulated in the curved vinyl plastisol and a cross-section of the coated yarn shows that where the core yarn is of multi-filament, the vinyl compound is interspersed among the individual filaments. Such a construction gives good adhesion to the core yarn; however, the major portion of the vinyl resin surrounds and encapsulates the core yarn. Normally, the vinyl content of the final thread ranges from 50 to 90% by weight thereof.
An important use for such a thread is in the preparation of fabrics, either woven or non-woven. A key feature in the process by which the fabrics are produced is that it must be possible to prepare the thread in cured form without activating the foaming agent therein. While it is possible to prepare a fabric from the thread after activation of the foaming agent, the process is uneconomical and the resultant fabric is inferior to the case wherein the process steps include curing of the thread without activation of the foaming agent prior to preparation of a fabric and activation of the foaming agent subsequent to manufacture of the fabric.
Richmond in U.S. Pat. No. 3,100,926 has described such a fabric wherein the thread is prepared in an extrusion process from a mixture of a thermoplastic resin and a blowing agent. The extruded thread is formed into a fabric-like material which is then thermally treated to decompose the blowing agent and evolve gases which have the dual effect of expanding the thermoplastic thread and welding the thermally-softened threads at juncture points in a permanent bond. Richmond shows both a non-woven fabric and a number of woven fabrics. In general, the woven fabrics are such that there is little if any open area so that such fabrics serve to screen a region completely from view, when used as curtains or area dividers. As is evident, it is the thickness of the thread in the plane of the fabric rather than in the transverse direction which is significant in determining the "opacity" or "shade-factor" of the fabric. If a thin thread is to be used to produce a desired degree of opacity then a large number of picks must be used. Conversely, if a thick thread is used then the quantity of encapsulating material necessary is high and the material cost is high.
A further difficulty with the process as disclosed by Richmond is that the extrusion temperature must be controlled within relatively narrow limits since any undue elevation of temperature during the extrusion can result in partial or complete activation of the foaming agent. Even more important, by the extrusion process, no more than a small number of threads such as 2 or 3 can be processed simultaneously by a single extruder due to the practical difficulties of dealing with yarn ends of finite length, breakage of the yarn and keeping the ends separate as they leave the single die. As is evident, then, the difficulties inherent in the extrusion process for preparing a fabric as described herein, both with respect to control of the process and with respect to the essentially cylindrical shape of the thread in its expanded form would make it desirable that a more effective and economical process and product be developed.